1. Field of the Invention
The present invention generally relates to wear-resistant castings and their manufacture and, more particularly, to articles having particles of sintered or cast hard carbides disposed in a casted steel alloy matrix, and to composite structures formed therefrom.
2. Description of the Prior Art
Parts for use in severe environments must combine wear resistance with toughness. Applications for such parts include earth or road engaging wear shoes, excavator teeth, and crusher teeth.
Suitable wear-resistant materials have been made of cemented carbide alloys consisting of a finely dispersed hard carbide phase cemented together by cobalt or nickel or both. The materials are produced by compacting finely milled powders together followed by liquid phase sintering to achieve consolidation. Typically the cemented carbide alloys possess microstructures characterized by hard carbide grains generally in the range of 1-15 microns. However, such materials may be subject to chipping or cracking when utilized by themselves. For those applications, it is desirable to have the wear properties of carbide combined with the toughness of steel.
The use of a cast iron or steel matrix as a binding material has proven difficult because the finely divided state and high specific surface of the dispersed hard carbide phases and the formation of comparatively brittle binder alloys of tungsten and iron with carbon. This reduces the free binder volume fraction of the body, thereby embrittling the sintered body. Unlike cobalt and nickel, the iron component of cast iron or steel will form a stable carbide (Fe3C) and has a greater tendency to form brittle binary carbides than either the cobalt or nickel binder materials. In addition, carbon transfer from the hard carbide phase or phases to the iron component is promoted by the presence of the liquid or plastic state of the iron or steel binder during liquid phase sintering when carried out at temperatures near to or above the melting point of the binder. However, useful wear resistant bodies have been made by casting a steel or cast iron melt into a bed of comparatively coarse hard carbide particulate.
One such technique is. set forth by the molten steel casting method of Charles S. Baum (U.S. Pat. Nos. 4,024,902 and 4,146,080). Unlike the prior art methods which had attempted to avoid the dissolution of the metallic carbide components into the matrixing alloy, Baum taught the placement of tungsten carbide particles of substantially larger size than those desired in the finished article in a mold in which the wear resistant body is to be formed.
According to Baum, a steel alloy is separately heated and casted into the mold which is at a temperature below the temperature at which the metallic carbide dissolves. The size and placement of the particles are balanced with the temperature of the molten steel, the initial temperature of the mold, and the volume and surface area of the mold to insure that the heat of the molten steel causes a dissolving action at the surface of the particles and at least some of the particles still exist in reduced size when the molten steel freezes. The fusion of the carbon, tungsten and cobalt through the alloy also produces an alloy having superior strength, including greater strength than the original casted alloy. In addition, the degree of solubility may be controlled by the inclusion of some smaller sintered particles that totally dissolve as the molten metal solidifies.
Another such wear resistant body is disclosed in U.S. Pat. No. 4,119,459 issued to Ekemar. Ekemar found that cemented carbide could be bonded in a matrix of graphitic cast iron having a carbon equivalent in the range of from 2.5 to 6.0 weight percent (wt. %). Ekemar also found that a suitable adjustment of the particle size of the hard carbide gave the possibility to reach the desired relationship between completely transforming or partially transforming the hard carbide particles.
It would be expected that the wear resistant bodies formed by the molten steel casting method may have superior physical properties over similar molten-cast iron bodies. For example, martensitic ductile cast iron can result in tensile strengths of up to 120 ksi, which is considered high for ductile iron. However, medium carbon steels may have tensile strengths of up to 220 ksi. Thus, a matrix of low alloy steel will have approximately twice the strength of a comparable cast iron product. Furthermore, the hardness of heat treated, low alloy steel casting would be between 40 and 50 R.sub.c versus 38 R.sub.c for ductile iron.
However, wear-resistant bodies produced by either the molten-steel or the molten-cast iron casting methods are often not suitable when used solely as a stand-alone product because their high cost and brittleness. Instead, the wear-resistant body may be more cost effective when used to increase the wear-performance of a larger steel casting in which it is incorporated.
It has been relatively easy to incorporate Wear resistant bodies produced by the molten-cast iron method into larger steel castings. For example, U.S. Pat. No. 4,584,020, issued to Waldenstrom, discloses a technique for incorporating a wear resistant molten-cast iron and carbide insert in a larger steel casting. The technique consists of applying between the casted steel alloy and the wear resistant insert a layer or zone of another metallic material with a higher toughness than the cast alloy. Generally the metallic material also has a higher melting point than the cast alloy and preferably at least 200 to 400 degrees C. (360 degrees F. to 720 degrees F.) above the melting point of the cast alloy. The metallic material is formed from a low carbon steel having a carbon content of 0.2% at the most. The thickness of the sheet of low carbon steel is at least 0.5 mm and preferably 1 to 8 mm.
Unfortunately, problems have arisen when attempting to incorporate molten-steel wear resistant bodies in larger castings. Several approaches have been tried to overcome these problems. E. L. Furman et al ("Reinforcing Steel Castings With Wear-Resisting Cast Iron," Liteinoe Proizvodstvo, No 7, p 27 (1986)) found that wear resistant bodies could be successfully incorporated into larger steel castings when the steel was poured at between 1450 to 1480 degrees C. (2642 to 2696 degrees F.). However, when the steel pouring temperature was raised above 1500 degrees C. (2732 degrees F.). it caused hot tearing and shrinkage blow holing inside the wear resistant inserts. Furman found that more effective reinforcement could be achieved by coating the inserts with a low melting brazing alloy, such as pure copper, prior to pouring the mold. Upon pouring, the copper brazing alloy melts and wets the surfaces of the inserts and the poured steel. A suitable fluxing agent was incorporated to prevent oxidation or the inserts during pouring.
U.S. Pat. No. 4,608,318, issued to Makrides et al discloses a tough, wear resistant composite. Carbide particles and a stainless steel metallic matrix are first formed into a wear-resistant insert by powder metallurgical methods including blending the powders, isostatically compacting the blend, and consolidating to form the insert. A second metallic matrix of molten metal is then bonded to the wear-resistant insert to complete the composite. The second metallic matrix formed by the molten metal may be a ferrous or non-ferrous alloy and is preferably steel.
Another powder metallurgical approach to this problem is disclosed in Australian Patent No. AU-B1-31362/77. According to the background discussion in U.S. Pat. No. 4,608,318, the Australian reference teaches milling a heat treatable low alloy steel powder together with a tungsten carbide or tungsten molybdenum solid solution carbide powder and then pressing and sintering to form the wear-resistant insert. Low alloy steel is then cast about the sintered wear-resistant insert to form the finished composite.
Certain disadvantages become apparent with the prior art. First, the technique as taught by Furman requires the additional step of coating the individual inserts. This method not only increases the cost of the final composite body but also creates an additional interface which may result in a later failure. Second, the powder metallurgical methods taught by Makrides and also Australian patent No. AU-B1-31362/77 are significantly more costly due to the necessary steps of preparing milled powders, blending, and isostatically pressing to form the insert.
It has thus become desirable to develop a wear-resistant "cast carbide/steel composite" insert having the strength and hardness advantages achieved by using a molten steel casting alloy and, at the same time, eliminating the prior art problems of hot tearing and shrinkage when the wear resistant body is incorporated into a larger steel casting.